In an uncooled infrared image sensing device according to the related art, a sharp infrared image is obtained by exercising precise temperature control using an electronic cooling unit for the purpose of minimizing non-uniformity of a microbolometer array.
As an electronic cooler is costly (and complicated), system cost (and complexity) rises. Accordingly, a new technique is required in order to deal with the characteristics and non-uniformity of the microbolometer array over a broad range of temperatures.
The performance of a microbolometer array declines owing to a variation in response between individual microbolometer detectors responsive to an uniform incident infrared radiation.
Causes of such variation that can be mentioned include the infrared absorption coefficient, resistance, TCR (Temperature Coefficient of Resistance), thermal capacity and coefficient of heat-transfer of the individual detectors.
Since the scope of a variation in response caused by such non-uniformity can become greater than the magnitude of the actual response responsive to an incident infrared radiation, it is usually necessary to apply various techniques in order to compensate for non-uniformity and obtain a signal that corresponds to the incident infrared radiation.
In the usual microbolometer array, output voltage produced by each microbolometer varies greatly depending upon substrate temperature. There may also be cases where average output voltage in several microbolometers included in the array falls outside the range of minimum and maximum signals, as a result of which a satisfactory FPA (Focal Plane Array) performance within a desired range of operating temperatures is not obtained.
For example, as illustrated in FIG. 1, the output voltage of a microbolometer in a microbolometer may fall below the minimum dynamic range of the system before it reaches a maximum desired substrate temperature. Alternatively, as illustrated in FIG. 2, the output voltage of another microbolometer in the microbolometer array may rise above the maximum dynamic range of the system before it reaches the maximum desired substrate temperature (see Patent Document 2).
A method of temperature compensation for mitigating such non-uniform behavior in a desired temperature range has been disclosed. For example, in Patent Document 1, as opposed to two-point non-uniformity correction in the related art, the factor of substrate temperature is taken into consideration, bias voltage applied to the bolometer is adjusted in such a manner that the sensitivity of incident infrared radiation will be constant even if the substrate temperature changes, and a correction is applied that will result in a uniform output voltage characteristic.
FIG. 3 illustrates a simplified circuit for applying a temperature compensation, and FIG. 4 illustrates a signal flow for applying a temperature compensation (see Patent Document 1).
The temperature compensation process disclosed in Patent Document 1 is as follows:
(1) Sensor output is measured under a condition of two incidence levels and two substrate temperatures. Here note is taken of outputs from two pixels. The result is shown in FIG. 5A.
(2) Sensitivity responsive to the incident infrared radiation differs between two pixels at two substrate temperatures. Further, mean gain is calculated at each substrate temperature. The result is shown in FIG. 5B.
(3) The bias voltage of each pixel is adjusted in such a manner that the mean gains at each of the substrate temperatures will be equal. The result is shown in FIG. 5C.
(4) A conventional two-point correction, namely a gain correction and an offset correction responsive to incident infrared radiation, is carried out. The result is shown FIG. 5D.
FIG. 6 illustrates the result of the correction by the series of steps (1) to (4) above.
A spatially substantially uniform distribution is obtained between incidence levels Qmin, Qmax and substrate temperatures Tmin, Tmax.
FIG. 7 is a schematic view of an on-chip readout circuit. An action that will adjust bias voltage applied to a bolometer in such a manner that the sensitivity of incident infrared radiation will be rendered constant even if substrate temperature changes is implemented by a DAC 36, and a conventional offset correction with respect to incident infrared radiation is performed by a DAC 74.
FIG. 8 illustrates an example of a system configuration that includes a readout circuit.
Further, Patent Document 2 discloses a circuit in which relative mismatch between the temperature coefficient of resistance (TCR) of an active microbolometer 3 and that of a reference microbolometer 2 shown in FIG. 9 is compensated for by providing a variable resistor 26 in series with the active microbolometer 3. The variable resistor 26 can be calibrated over the desired temperature range to minimize the effects of the relative mismatch. In addition, the circuit of FIG. 9 also includes DAC (digital-analog converter) 40 and offset resistor 38.
For example, with reference to FIG. 9, in a case where the relative mismatch between the active microbolometer 3 and the reference microbolometer 2 is such that as substrate temperature rises, the resistance of the active microbolometer 3 decreases at a rate faster than the reference microbolometer 2, output voltage 42 will increase as the substrate temperature rises for a given level of incident infrared radiation. This is represented by curve 51 (FIG. 10) with respect to a minimum resistor value for resistor 26.
If, when measurements are repeated over the same substrate temperature range, the resistance value of variable resistor 26 shown in FIG. 9 is increased and the offset is adjusted so that the output voltage 42 is returned to the initial value for a minimum substrate temperature, then the output voltage 42 will increase at a lower rate relative to a rise in temperature. This is represented by curve 52. This process can be repeated for various values of the variable resistor 26, and curves 53 and 54, for example, are obtained. As is clear from curves 51 to 54, the curve 54 provides the best response over the desired substrate temperatures. Furthermore, this process can be used to obtain optimum resistor settings for each microbolometer in the array and obtain a characteristic of the kind shown in FIG. 11.
Further, the following is set forth in paragraph [0026] of Patent Document 3:
The influence of background radiation related to ambient temperature in the vicinity of a package housing a sensor is one factor that causes a change in the output voltage of the sensor. This influence contributes to an average (i.e., DC) component of signals from the sensor. Normally this influence essentially varies with time. This influence can be compensated for by an imaging system, for example, by subtracting it from the signals.    [Patent Document 1] WO 98/35212    [Patent Document 2] Japanese Patent Kohyo Publication No. JP-P2005-519266A    [Patent Document 3] Japanese Patent Kohyo Publication No. JP-P2003-532111A